The present invention relates to a source driver, an electro-optical device, an electronic instrument, and the like.
As a liquid crystal panel (electro-optical device) used for electronic instruments such as portable telephones, a simple matrix type liquid crystal panel and an active matrix type liquid crystal panel using a switching element such as a thin film transistor (hereinafter abbreviated as “TFT”) are known.
The simple matrix method can easily reduce power consumption as compared with the active matrix method. On the other hand, it is difficult to increase the number of colors or display a video image using the simple matrix method. The active matrix method is suitable for increasing the number of colors or displaying a video image, but has difficulty in reducing power consumption.
In recent years, an increase in the number of colors and a video image display have been increasingly demanded for portable electronic instruments such as portable telephones in order to provide a high-quality image. Therefore, the active matrix type liquid crystal panel has been increasingly used instead of the simple matrix type liquid crystal panel. In the active matrix type liquid crystal panel, a signal applied to a source line is written into a pixel selected by a gate line to change the transmissivity of the pixel.
In recent years, the number of source lines of a liquid crystal panel has been increased along with an increase in the screen size and the number of pixels of a liquid crystal panel. On the other hand, an increase in accuracy of the voltage applied to each source line has been demanded. Moreover, a reduction in power consumption of a source driver which drives source lines of a liquid crystal panel and a reduction in chip size of such a source driver have been demanded along with a demand for a reduction in weight and size of battery-driven electronic instruments provided with a liquid crystal panel. Therefore, a source driver is desired which has a simple configuration and a high performance.
For example, JP-A-2005-175811 and JP-A-2005-175812 disclose a configuration that enables a Rail-to-Rail operation of an output circuit of a source driver which drives a source line while enabling a voltage to be supplied to the source line with high accuracy.
According to the technologies disclosed in JP-A-2005-175811 and JP-A-2005-175812, the Rail-to-Rail operation is realized by controlling drive capability by providing an auxiliary circuit in each output circuit. Therefore, since it is necessary to provide the auxiliary circuit as an additional circuit, the circuit scale of the source driver increases. Moreover, the transistor size must be increased in order to suppress a variation in voltage applied to the source line.
Furthermore, in order to supply a voltage to the source line with high accuracy, a voltage from a DAC which generates a grayscale voltage corresponding to grayscale data must be directly supplied to the source line. Therefore, it is necessary to increase the number of grayscale voltage signal lines as the number of grayscales increases, whereby the chip size increases.
In general, an operational amplifier must be designed taking a variation in output voltage into consideration. Therefore, it is necessary to suppress a variation in output voltage by increasing the size of a transistor forming an operational amplifier.